1. Field of the Invention
The present invention relates to a display device such as a liquid crystal display device used for AV (audio and visual) apparatuses and OA (office automation) apparatuses, and a method for driving such a display device. More specifically, the present invention also relates to a display device incorporating dual-terminal nonlinear elements and a method for driving such a display device, in which appropriate corrections are performed corresponding to an ambient temperature around the device and corresponding to the characteristics of the elements.
2. Description of the Related Art
Recently, liquid crystal display devices are used for a variety of purposes, for example, as display devices for AV and OA apparatuses. Low-end apparatuses incorporate passive type liquid crystal display devices such as TN (twisted-nematic) display device and STN (super-twisted-nematic) display device. High quality apparatuses incorporate liquid crystal display devices driven according to an active matrix approach, in which three-terminal nonlinear elements represented by TFTs (thin-film-transistors) or dual-terminal nonlinear elements represented by MIMs (metal-insulator-metal) are used as switching elements.
Such liquid crystal display devices driven according to the active matrix approach provide thinner and lighter devices which have a color reproduction quality superior to that of CRTs (cathode-ray-tube), and reduced power consumption, and therefore use of such liquid crystal display devices is rapidly increasing. Using TFTs as switching elements, however, requires 6 to 8 steps of thin film forming processes and photolithography processes during production of the display devices. Therefore, solutions for cost reduction are highly sought.
On one hand, liquid crystal display devices incorporating dual-terminal nonlinear elements as switching elements have been developed rapidly since they are advantageous to TFTs in terms of cost and advantageous to passive type liquid crystal display devices in terms of display quality.
The dual-terminal nonlinear element is known for changing its characteristics depending on an ambient temperature. As illustrated in FIG. 2, when the ambient temperature increases, the characteristics of the dual-terminal nonlinear element change from the characteristics indicated by a solid line a to characteristics indicated by a dashed line b, i.e., to low resistance characteristics, and when the ambient temperature decreases, the characteristics of the dual-terminal nonlinear element change from the characteristics indicated by a solid line a to characteristics indicated by a dashed line c, i.e., to high resistance characteristics.
As the characteristics of the dual-terminal nonlinear element change depending on the ambient temperature, a voltage-transmittance characteristic of the display device incorporating the dual-terminal nonlinear element also changes depending on the ambient temperature. This means that the display condition of the display device changes depending on the ambient temperature, which is a fatal problem for display devices used in certain temperature ranges.
In order to improve such a temperature characteristic of the dual-terminal nonlinear element, studies has been made for obtaining a better material and a better structure for the dual-terminal nonlinear element, but no outstanding effects have as yet been achieved. Another solution is to incorporate an independent heating device or a cooling device together with the display device so as to maintain the display section at a constant temperature. This, however, increases the cost and the size of the apparatus.
As methods for improving the temperature characteristic by means of changing the manner in which the display is driven, it has been conventional to change a driving voltage value or a bias value applied to the liquid crystal. Regarding a display device incorporating dual-terminal nonlinear elements, for example, Japanese Laid-open Publication No. 5-53092 discloses a driving method in which a rate of change depending on the temperature is varied between a bias potential, a data amplitude voltage, and the maximum selection potential. In Japanese Laid-open Publication No. 5-53092, the maximum selection potential means the maximum potential during a selection period, the bias potential means the time average of the potential during a nonselection period, and the data amplitude voltage means the difference of the selected pulse between operation time and nonoperation time of the liquid crystal panel. A correction of the temperature is performed based on the following three criteria: (1) the bias potential is always at a non-zero value; (2) the temperature-dependent variation rate of the maximum selection potential is set to be greater than the temperature-dependent variation rate of the bias potential; and (3) the temperature-dependent variation rate of the data amplitude voltage is set to be smaller than the temperature-dependent variation rate of the maximum selection potential.
The above method, however, requires alteration of the bias potential during a nonselection period, and therefore requires a variety of potentials. It may also increase the power consumption since the energy loss in the power supply circuit is great due to the variable amplitudes.
Moreover, it is difficult in the above method to change the characteristics of dual-terminal nonlinear element so as to obtain display devices for specified uses, e.g., display devices emphasizing uniformity, display devices emphasizing contrast, or the like.
In one aspect of the invention, a method for driving a display device includes a pair of substrates disposed so as to face each other with a display medium inserted therebetween, one of the substrates including: a first wiring of either a plurality of scanning lines or a plurality of signal lines; and at least one dual-terminal nonlinear element which is connected to the first wiring and functions as a switching element for selecting matrix-like pixels, and the other substrate including a second wiring of the other of the plurality of scanning lines or the plurality of signal lines provided in a direction perpendicular to the first wiring, the method including the step of altering an amplitude of a supply voltage applied to each of the pixels during a selection period and an amplitude of a modulated voltage applied to each of the pixels during a nonselection period, corresponding to an ambient temperature level, wherein the amplitude of the supply voltage and the amplitude of the modulated voltage are decreased in a case where the ambient temperature increases, and the amplitude of the supply voltage and the amplitude of the modulated voltage are increased in a case where the ambient temperature decreases, and the rate of amplitude change of the modulated voltage to the change of the ambient temperature is greater than the rate of amplitude change of the supply voltage to the change of the ambient temperature.
In another aspect of the invention, a method for driving a display device includes a pair of substrates disposed so as to face each other with a display medium inserted therebetween, one of the substrates including: a first wiring of either a plurality of scanning lines or a plurality of signal lines; and at least one dual-terminal nonlinear element which is connected to the first wiring and functions as a switching element for selecting matrix-like pixels, and the other substrate including a second wiring of the other of the plurality of scanning lines or the plurality of signal lines provided in a direction perpendicular to the first wiring, the method including the step of altering a pulse width of a supply voltage applied to each of the pixels during a selection period and a pulse width of a modulated voltage applied to each of the pixels during a nonselection period, corresponding to an ambient temperature, wherein the pulse width of the supply voltage and the pulse width of the modulated voltage are decreased in a case where the ambient temperature increases, and the pulse width of the supply voltage and the pulse width of the modulated voltage are increased in a case where the ambient temperature decreases.
In another aspect of the invention, a method for operating a display device includes a pair of substrates disposed so as to face each other with a display medium inserted therebetween, one of the substrates including: a first wiring of either a plurality of scanning lines or a plurality of signal lines; and at least one dual-terminal nonlinear element which is connected to the first wiring and functions as a switching element for selecting matrix-like pixels, and the other substrate including a second wiring of the other of the plurality of scanning lines or the plurality of signal lines provided in a direction perpendicular to the first wiring, the method including the step of altering an amplitude of a supply voltage applied to each of the pixels during a selection period and a pulse width of a modulated voltage applied to each of the pixels during a nonselection period, corresponding to an ambient temperature level, wherein the amplitude of the supply voltage and the pulse width of the modulated voltage are decreased in a case where the ambient temperature increases, and the amplitude of the supply voltage and the pulse width of the modulated voltage are increased in a case where the ambient temperature decreases.
In another aspect of the invention, a method for driving a display device includes a pair of substrates disposed so as to face each other with a display medium inserted therebetween, one of the substrates including: a first wiring of either a plurality of scanning lines or a plurality of signal lines: and at least one dual-terminal nonlinear element which is connected to the first wiring and functions as a switching element for selecting matrix-like pixels, the other substrate including a second wiring of the other of the plurality of scanning lines or the plurality of signal lines provided in a direction perpendicular to the first wiring, the method including the step of altering a pulse width of a supply voltage applied to each of the pixels during a selection period and an amplitude of a modulated voltage applied to each of the pixels during a nonselection period, corresponding to an ambient temperature level, wherein the pulse width of the supply voltage and the amplitude of the modulated voltage are decreased in a case where the ambient temperature increases, and the pulse width of the supply voltage and the amplitude of the modulated voltage are increased in a case where the ambient temperature decreases.
In another aspect of the invention, a method for driving a display device includes a pair of substrates disposed so as to face each other with a display medium inserted therebetween, one of the substrates including: a first wiring of either a plurality of scanning lines or a plurality of signal lines; and at least one dual-terminal nonlinear element which is connected to the first wiring and functions as a switching element for selecting matrix-like pixels, the other substrate including a second wiring of the other of the plurality of scanning lines or the plurality of signal lines provided in a direction perpendicular to the first wiring, wherein the amplitude of the supply voltage and the amplitude of the modulated voltage are altered corresponding to a current/voltage characteristic of the at least one dual-terminal nonlinear element, and wherein the amplitude of the supply voltage and the amplitude of the modulated voltage are increased in a case where the current/voltage characteristic of the dual-terminal nonlinear element is set to a high resistance current/voltage characteristic, and the amplitude of the supply voltage and the amplitude of the modulated voltage are decreased in a case where the current/voltage characteristic of the at least one dual-terminal nonlinear element is set to a low resistance current/voltage characteristic.
In one embodiment of the invention, the amplitude of the modulated voltage is in a range from 5 V to 15 V in a case where the current/voltage characteristic of the at least one dual-terminal nonlinear element is set so that a voltage value of the current/voltage characteristic is in a range from 5 V to 15 V at a current value of the current/voltage characteristic in a range of 1xc3x9710xe2x88x9210 A to 1xc3x9710xe2x88x928 A, and the amplitude of the modulated voltage is in a range from 1 V to less than 5 V in a case where the current/voltage characteristic of the at least one dual-terminal nonlinear element is set so that the voltage value of the current/voltage characteristic is in a range from 1 V to less than 5 V at the current value of the current/voltage characteristic in a range from 1xc3x9710xe2x88x9210 A to 1xc3x9710xe2x88x928 A.
In another embodiment of the invention, the at least one dual-terminal nonlinear element has a MIM structure.
Functions of the present invention will now be described.
According to the present invention, by individually controlling a correction waveform for a voltage applied to the scanning lines and a correction waveform for a voltage applied to the signal lines, it is possible to change, corresponding to the ambient temperature and the characteristics of a dual-terminal nonlinear element, the amplitude of a supply voltage applied to each pixel during a selection period and the amplitude of a modulated voltage applied to each pixel during a nonselection period.
The supply voltage applied during the selection period provides a voltage value capable of sufficiently charging a display medium (a liquid crystal layer) of each pixel, corresponding to the current-voltage characteristics (the I-V characteristics) of the dual-terminal nonlinear element. The modulated voltage applied during the nonselection period determines a maintenance characteristic. A synthesized voltage of the supply voltage and the modulated voltage determines the ON/OFF state of the display.
Since the I-V characteristics of the dual-terminal nonlinear element change corresponding to a temperature change, by changing the supply voltage and the modulated voltage corresponding to the change of the I-V characteristics, it is possible to achieve a sufficient correction effect of the I-V characteristics corresponding to the temperature change.
For example, in the case where the ambient temperature increases, the resistance characteristic of the dual-terminal nonlinear element decreases as shown in FIGS. 1A and 1B. Therefore, the amplitude of the supply voltage and amplitude of the modulated voltage are both made to decrease. In the case where the ambient temperature decreases as shown in FIGS. 1A and 1B, the resistance characteristic of the dual-terminal nonlinear element increases. Therefore, the amplitude of the supply voltage and amplitude of the modulated voltage are both made to increase. In both FIGS. 1A and 1B, the current value is constant (e.g., I=1xc3x9710xe2x88x926 A in FIG. 1A and I=1xc3x9710xe2x88x9211 A in FIG. 1B) while the I-V characteristics of the dual-terminal nonlinear element corresponding to the ambient temperature changes. As shown in these graphs, if the current value is constant, the display characteristic of the display medium (the liquid crystal layer) corresponding to the temperature change becomes substantially constant. In order to make the current value constant regardless of the ambient temperature change, the supply voltage and the modulated voltage have to be independently changed. Furthermore, since the voltage change regarding to the change of I-V characteristics corresponding to the temperature is greater in the case where the current value is low than in the case where the current value is high as shown in FIGS. 1A and 1B, the rate of amplitude change for the modulated voltage is made greater than that for the supply voltage, corresponding to the ambient temperature.
The I-V characteristics of the dual-terminal nonlinear element, having for example, an MIM structure, can be changed depending on the structure or the materials used, or the temperature conditions during manufacturing. The supply voltage and the modulated voltage can be changed corresponding to the change of the I-V characteristics. Therefore, it is possible to obtain a variety of display characteristics suitable for each specific use. For example, a display emphasizing uniformity, a display emphasizing contrast, or the like is obtained.
In the case where the resistance characteristic is that of a high resistance among the I-V characteristics of the dual-terminal nonlinear element, a high duty and high contrast display characteristic can be obtained by increasing the amplitude of at least one of the supply voltage or the modulated voltage. For example, if the dual-terminal nonlinear element has current-voltage characteristics in which the voltage value is in a range from 5 V to 15 V at the current value in a range from 1xc3x9710xe2x88x9210 A to 1xc3x9710xe2x88x928 A, it is preferable to set the amplitude of the modulated voltage in a range from 5 V to 15 V. In the case where the resistance characteristic is low among the I-V characteristics of the dual-terminal nonlinear element, a display characteristic, in which the contrast in the display screen is uniform even at a low driving voltage, can be obtained by decreasing the amplitude of at least one of the supply voltage or the modulated voltage of the dual-terminal nonlinear element. As a result, it is possible to reduce the device cost by using driving members which have a low level voltage tolerance. For example, if the dual-terminal nonlinear element has current-voltage characteristic in which the voltage value is in a range from 1 V to less than 5 V at the current value in a range from 1xc3x9710xe2x88x9210 A to 10xe2x88x928 A, it is preferable to set the amplitude of the modulated voltage in a range from 1 V to less than 5 V.
Thus, the invention described herein makes possible the advantages of (1) providing a display device which is capable of preventing deterioration of the display quality due to an ambient temperature change, and selecting a suitable driving condition corresponding to changes in the characteristics of a dual-terminal nonlinear element, thereby achieving display characteristics suitable for each specific use; and (2) providing a method for driving the display device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.